Algae growth-repellent underwater coatings are known, for the purposes of preserving objects which are exposed to water such as the hulls of boats and the like, in the form of the coatings referred to as anti-fouling paints which, for preventing algae growth from occurring on such objects, have toxic, generally inorganic compounds or color pigments such as lead and mercury compounds. Such pigments pollute the water of seas and rivers due to their being rubbed off.